1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically to a semiconductor integrated circuit having a function of preventing a malfunction caused by noise.
2. Description of Related Art
In general, high speed operation and a high integration of a semiconductor integrated circuit are being advanced with demands for a high speed operation and a high integration of a system because of advanced information processing technology.
Recently, semiconductor integrated circuit have been used in a variety of devices, apparatuses, and machine because of advanced information processing technology. For example, semiconductor integrated circuits are now widely used not only in in-house machines such as rice cookers and electric washing machines, but also in large machines encountered in daily life such as automobiles, signals, and crossing gates. At present, the semiconductor integrated circuits themselves are deeply integrated into daily activity and have become non-detectable to a human being.
The reason that the semiconductor integrated circuits are widely utilized in human activity is that machines and apparatuses incorporating the semiconductor integrated circuits, typified by a microcomputer in most cases, have intelligence, so that a comfortable environment is realized by the intelligent machines and apparatuses.
On the other hand, because the semiconductor integrated circuit is incorporated in a machine, accidents have happened. For example, it was reported that an accident resulting in injury or death occurred because a semiconductor integrated circuit constituting a program controlled device for automatic speed control in an automobile having an automatic transmission caused a program runaway for an unknown reason. This is an example indicating that the semiconductor integrated circuits deeply integrated in human activity must have high reliability and high operational accuracy.
In most cases, however, environments in which the semiconductor integrated circuits are actually used in various machines have large temperature changes, moisture changes, and are filled with electrical noise and mechanical vibration. These conditions make it difficult to achieve high reliability and high operation accuracy. In addition, these are common operating environments of the semiconductor integrated circuits used for engine control in automobile and aircraft.
Therefore, in attempts to design a semiconductor integrated circuit to be used in a very severe environment as mentioned above, the circuit is designed with high reliability and high operational accuracy of the circuit as top priorities. As a result, a high speed operation of the circuit is sacrificed.
Since the conventional semiconductor integrated circuits are exposed to electrical noises as mentioned above, Japanese Patent Application Laid-open Publication No. JP-A-367945, for example, proposed a circuit to detect a noise in a signal line so as to put a microprocessor into a standby condition when the noise is detected and to maintain the microprocessor into the standby condition until the noise is not detected in order to ensure that the microprocessor does not mistake the noise for a signal. In addition. Japanese Patent Application Laid-open Publication No. JP-A-1-20438 proposed to trigger a system setting when a noise is detected which cannot be sufficiently attenuated by a noise attenuation circuit in a program controlled apparatus in order to avoid a program runaway.
These conventional semiconductor integrated circuits have been manufactured with a MOS transistor gate length on the order of 1 .mu.m and a MOS transistor threshold on the order of 0.7 V (in N-channel MOS transistor). Accordingly, the above mentioned methods are effective in semiconductor integrated circuits which cause a simple control program to run with a clock on the order of 10 MHz.
However, semiconductor integrated circuits generally used in control systems are not only required to have high reliability and a highly accurate operation, but are now required to meet requirements for high speed operation and complicated operation. This demand for high speed operation and complicated operation is now increasing rapidly.
As a result, semiconductor integrated circuits incorporated in control systems can no longer be accepted if they cannot realize the high speed operation and the complicated operation similar to semiconductor integrated circuits used in information processing system such as EWS, and apparatuses incorporating a semiconductor integrated circuit.
Under this background, an effort for increasing the operational speed of semiconductor integrated circuits incorporated in general control systems has been advanced in a manner similar to that made in the semiconductor integrated circuits used in information processing systems. Namely, in accordance with a scaling rule in a design standard of the semiconductor process, efforts have been made to microminiaturize the semiconductor integrated circuits and to lower the supply voltage and the threshold of transistors and also lower the temperature of operating environment.
For example, the gate length of N-channel MOS transistors has been shorted to 0.35 .mu.m from 1 .mu.m in the prior art, and the supply voltage has been lowered from 5 V to 3 V. In addition, the threshold of MOS transistors has been lowered from 0.7 V to 0.4 V. With these improvements, the semiconductor integrated circuits have been constituted of N-channel MOS transistors intended for high speed operation.
However, it has been known that the above mentioned high speed N-channel MOS transistors has poor resistance to noise on a ground (GND) line and a supply voltage (VDD) line and to a rapid change in the supply voltage in comparison with the prior art MOS transistors, and therefore, the transistors are easily broken. This is because the supply voltage and the transistor threshold have been lowered, but the noise on the GND line and the VDD line and the rapid change in the supply voltage are essentially unchanged. As a result, the noise and the rapid change of the supply voltage having a conventional magnitude becomes large in comparison with the reduced supply voltage and the reduced threshold. Thus, a state held by a semiconductor integrated circuit constituted of the high speed MOS transistors is easily flipped by noises that had not posed a problem in the prior art semiconductor integrated circuit.
Because of this, a malfunction of a semiconductor integrated circuit constituted of the high speed MOS transistors cannot be completely prevented by a malfunction preventing circuit disclosed in Japanese Patent Application Laid-open Publication No. JP-A-4-367945 which is configured to maintain a microprocessor into a standby condition when there is a possibility of a malfunction until the possibility of the malfunctional disappears. In this connection, it may be considered to apply a substrate bias in the standby condition to evaluate the threshold voltage of the MOS transistors included in the semiconductor integrated circuit for the purpose of preventing the malfunction. However, since a substrate biasing circuit ceaselessly consumes electric power in the standby period, the power consumption in the standby condition becomes large in comparison with the conventional semiconductor integrated circuit.
Examine a system proposed by Japanese Patent Application Laid-open Publication No. JP-A-1-20438 to trigger a system setting for preventing the malfunction of the semiconductor integrated circuit. A program for controlling the semiconductor integrated circuit becomes complicated year after year, and therefore, the number of program steps from the system reset until the system start also increases. Accordingly, the method of No. JP-A-1-20438 becomes difficult to apply except in limited applications.
As mentioned above, in the semiconductor integrated circuits required to have high reliability and a high accurate operation, the malfunction caused by the GND noise and VDD noise generated by the semiconductor integrated circuits themselves or by external disturbance has become a large problem.
In particular, in the case that the semiconductor integrated circuits are constituted of MOS transistors having a reduced threshold for meeting the demands of the device size reduction and the supply voltage reduction so as to realize a high speed operation and a high integration degree, the malfunction occurrence percentage in logic gate circuits and other circuits constituted of low threshold MOS transistors, caused by the GGND noise, the VDD noise and the rapid variation in the supply voltage, is higher than the malfunction percentage in logic gate circuits and other circuits constituted of conventional high threshold MOS transistors. The malfunction rate is higher regardless of whether a circuit system is static or dynamic.
As a result, even if a static circuit is put into a standby condition for suppress malfunction which might be caused by the GND noise, the VDD noise and the rapid variation in the supply voltage, since a noise level similar to the conventional circuit is relatively large in comparison with the reduced threshold of the low threshold transistor, the circuit's state holding capability is weakened. Accordingly, the state before the circuit is put into the standby condition cannot be held by only the movement to the standby condition. As a result, a malfunction occurs when the circuit is returned from the standby condition.